1. Field of the Invention
The present invention relates to a method of manufacturing GaN crystals and a GaN crystal substrate, GaN crystals and a GaN crystal substrate obtained by the method, and a semiconductor device including the same.
2. Related Background Art
Gallium nitride (GaN) has been gaining attention as a material for semiconductor elements that emit blue or ultraviolet light. A laser diode (LD) that emits blue light is used for high-density optical disk devices or displays while a light emitting diode (LED) that emits blue light is used for displays, lighting, etc. It is expected to use an ultraviolet LD in the field of, for instance, biotechnology and an ultraviolet LED as, for example, an ultraviolet source for a fluorescent lamp.
Usually, GaN crystals that are used for LDs or LEDs are formed by heteroepitaxially growing GaN crystals on a sapphire substrate using vapor phase epitaxy. Generally, GaN crystals obtained by this method have a dislocation density of 108 cm−2 to 109 cm−2 and therefore have a problem in quality. For example, an epitaxial lateral overgrowth (ELOG) method has been developed as the method of solving this problem. With this method, the dislocation density can be reduced to about 105 cm−2 to 106 cm−2. This method, however, has a problem of its processes being complicated.
On the other hand, besides the vapor phase epitaxy, a method of growing crystals from a liquid phase also has been studied. However, since the equilibrium vapor pressure of nitrogen is at least 10000 atm (10000×1.013×105 Pa) at the melting point of GaN crystals, conventionally severe conditions including 8000 atm (8000×1.013×105 Pa) and 1200° C. are required for growing GaN crystals from a liquid phase. In order to solve this problem, a method using a Na flux has been developed. This method allows GaN crystals to be obtained under relatively mild conditions. Furthermore, a mixture of Ga and Na is pressurized and heated to be melted in a nitrogen gas atmosphere containing ammonia, then crystals are grown for 96 hours using the melt, and thereby single crystals whose maximum crystal size is about 1.2 mm have been obtained (see, for instance, JP2002-293696A).
Moreover, another method also has been reported in which a GaN crystal layer is formed on a sapphire substrate by a metalorganic chemical vapor deposition (MOCVD) method and then single crystals are grown by a liquid phase epitaxy (LPE) method.
These conventional methods, however, did not allow high quality GaN crystals to be manufactured under mild conditions.